From the earliest days of gas liquefaction, the thermodynamic process advantage of employing cryogenic liquefied gas expanders in place of throttling valves was very well recognized. However, the available technology was unable to offer reliable cryogenic liquefied gas expanders until the aerospace industry developed materials and designs suitable for the cryogenic environment.
Liquefied natural gas (“LNG”) expanders reduce the high pressure of the condensed liquefied gas by converting the static pressure energy of the fluid into electricity and sub-cooling the refrigerated LNG. The Carnot efficiency of the liquefaction process is significantly increased by using LNG expanders, resulting in a very short amortization time of less than six months for the financial investment in LNG expanders.
Modern process plants for the liquefaction of natural gas operate at high pressure to improve the overall efficiency of the cryogenic process. Following the condensation of the refrigerated gas, the pressurized LNG is expanded to a lower pressure suitable for storage and transportation. The expansion process generates some vapor and cools the remaining liquid. The aim of using an expander rather than a Joule-Thomson valve is to increase the amount of liquid and to decrease the amount of vapor at the outlet of the expander. By employing a two-phase expander with draft tube at the exit, an increased amount of liquid is produced in a near-isentropic expansion process. The benefits of this process are well documented, for example “Fifteen Years of Field Experience in LNG Expander Technology” Proceedings of the First Middle East Turbomachinery Symposium, Feb. 13-16, 2011, Doha, Qatar; and “Thermo-Fluid Dynamics and Design of Liquid-Vapour Two-Phase LNG Expanders” presented February 2010 at the Gas Processors Association, Europe, both hereby incorporated by reference.
However despite the benefits provided by the two-phase expander, there are unmet needs in the existing technology. For example, performance of prior art expanders is limited to certain flow rates and differential pressures, which can change over time as the fluid and gas from a well is discharged. As the flow rate decreases the efficiency decreases, such that when the flow rate decreases below 50%, the prior art expanders stop producing power and instead begin to consume power.
To address the need to change flow rates and differential pressures, it is known to exchange the nozzle ring that provides an entrance for the pressurized liquefied gas within a pressure containment vessel into a hydraulic assembly contained within the vessel with a different nozzle ring. Such nozzle rings have a plurality of fixed position guide vanes that direct the pressurized liquefied gas into the hydraulic assembly. By changing the position of the guide vanes, it is possible to change the flow rate and differential pressure. To change the position of the guide vanes, however, it is necessary to shut down the expander so as to take the expander apart to remove the existing nozzle ring and install the new one. Shutting down the expander for such purposes impacts production of the well and is costly. The present disclosure provides improvements and benefits to the prior art.